1. Field of the Invention
This invention relates to a method of improving sliding surfaces of an iron or steel base material of machineries in anti-abrasion property and anti-seizing property.
2. Description of the Related Art
Some piston pins of vehicle engines, for example, are used in slide fitting with pistons or connecting rods via bearing bushes. However, for the purpose of reducing parts and eliminating the steps of press-fitting the bushes, various methods have been proposed to omit bearing bushes and instead improve piston pins themselves in anti-abrasion property. There are, for example, a method of plating the surfaces (outer surfaces), as slide surfaces, of piston pins with chromium, and a method of treating piston pins by shot peening after carburizing.
There is also a method of treating chromium-plated cylinders and piston rings by electrolytic etching to form groove-like oil retainers and thereby prevent seizure, as disclosed in, for example, Japanese Patent Publication No. hei 7-15158.
Piston pins plated merely with chromium are certainly improved in anti-seizing property in relation to connecting rods. In relation to pistons, however, it is known that they are inferior to standard piston pins subjected to carburizing. Additionally, experimental data shows that the conventional method combining carburizing and shot peening can only attain anti-seizing property equivalent to or inferior to the property obtained by chromium plating alone. Furthermore, the method in which chromium plating and electrolytic etching for producing lubricant storing recesses are combined is subjected to serious abrasion of the grinder and takes a long time for the processing. If such method is employed to piston pins not supplied with enough lubricant from around, then the lubricant is liable to escape into groove-like lubricant retainers and break the lubricant film.
To overcome those problems, the invention provides a method of improving a slide surface of an iron or steel base material in anti-seizing property, comprising: processing the slide surface of the iron or steel material by carburizing; plating the slide surface subjected to the carburizing with chromium; and giving impulses to the slide surface subjected to the plating to produce fine cavities in the form of depressions of the chromium-plating layer and relatively large cavities produced by exfoliation of the chromium-plating layer on the slide surface.
The invention, having the above-summarized configuration and giving impulse to the chromium-plated slide surface, ensures that lubricant is held in the lubricant-retaining cavities formed in the chromium-plating layer by the impulses and prevents seizure of the slide surface. The fine-cavity lubricant retainers initially provide a good lubrication between the slide surface and the mating surface while seldom causing seizure, and when these fine cavities in the form of depressions of the chromium-plating layer are lost due to the surface wear of the chromium-plating layer, the relatively large cavities produced by exfoliation of the chromium-plating layer remain as lubricant retainers and maintain the anti-seizing property for a long time. In addition, since the invention makes the larger cavities by exfoliation and not by plastic deformation, it produces almost no projections (burrs or swellings), and does not need finish machining after shot peening.
In the present invention, each fine cavity preferably has a size not larger than 5 xcexcm whereas each larger cavity preferably has a size in the range from 5 xcexcm to 200 xcexcm to ensure more reliable effects of the invention.
The impulses can be given by shot peening. In this case, glass beads are typically used as peening elements.
The diameter of the beads is preferably in the range of from #120 to 240. It has been found that if the shot peening is carried out with an air pressure from 4.0 to 5.0 kgf/cm2 and at a projection distance of from 20 to 50 mm, the best result will be obtained.